Internal combustion engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. This operation may be referred to as VDE (variable displacement engine) operation. In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as engine speed and/or load thresholds, as well as various other operating conditions such as vehicle speed. A control system may enable and/or disable selected cylinders through adjustment of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves.
Each cylinder valve deactivator may be a rolling finger follower of a deactivatable valve assembly, with each rolling finger follower being switchable from an activated mode to a deactivated mode (and vice versa). During conditions in which a rolling finger follower is in the activated mode, an outer arm of the roller finger follower is driven by rotation of a cam of a camshaft to move a poppet valve, with the movement of the poppet valve controlling intake of gases into a combustion chamber of the engine or controlling flow of exhaust gases out of the combustion chamber. In the deactivated mode, the outer arm is not driven by the cam so that the rotational motion of the cam is not translated to the poppet valve, thereby resulting in a lost motion.
However, the rolling finger followers of the deactivatable valve assembly are often produced with inherent nominal lash and lash maximum wear characteristics that are different than non-deactivatable rolling finger followers. These characteristics may result in different amounts of lift and/or a different lift timing of poppet valves being driven by the deactivatable rolling finger followers. One example approach to address these issues is shown by Hendriksma et al. in U.S. Pat. No. 7,322,329. Therein, a valve-deactivation roller hydraulic valve lifter assembly process includes associating leakdown test results for individual lash adjusters with residual lash test results to minimize total length variation in the deactivation roller hydraulic valves. Another example approach is shown by Hicks in U.S. Pat. No. 6,513,471. Therein, a timing of exhaust cams driving valves of deactivatable cylinders is advanced relative to a timing of exhaust cams driving valves of non-deactivatable cylinders. This results in an amount of overlap of opening time of valves of the deactivatable cylinders to be approximately a same amount of overlap as valves of the non-deactivatable cylinders.
However, the inventors herein have recognized potential issues with such systems. As one example, reducing the length variation between valve-deactivation roller hydraulic valve lifters may reduce an amount of variation in lift and/or lift timing of poppet valves driven by the lifters, but it does not address the issue of differences in lift and/or lift timing of deactivatable poppet valves relative to non-deactivatable poppet valves. As another example, advancing a timing of cams associated with deactivatable valves relative to cams associated with non-deactivatable valves may increase a control complexity of the engine and reduce engine efficiency.
In one example, the issues described above may be addressed by a system, comprising: a camshaft including first and second pluralities of cams, each cam of the first plurality of cams having a first cam lobe profile, and each cam of the second plurality of cams having a different, second cam lobe profile; a plurality of deactivatable cylinder valves driven by the first plurality of cams; and a plurality of non-deactivatable cylinder valves driven by the second plurality of cams. In this way, each of the deactivatable cylinder valves and non-deactivatable cylinder valves may have a same valve opening rate and valve closing rate, and a same amount of valve overlap.
As one example, each cam of the first and second pluralities includes an outer surface tapering from a base section of the cam to a nose of the cam. The outer surface of each cam of the first plurality of cams has a different curvature than the corresponding outer surface of each cam of the second plurality of cams. By configuring the cams in this way, the second plurality of cams drives the non-deactivatable cylinder valves with a same timing and lift amount as the deactivatable cylinder valves driven by the first plurality of cams, and by driving the valves with the same timing and lift amount, combustion stability of an engine including the cams and cylinders may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.